Thanks for your comments & suggestions.
With the Cron running regularly, things seem much better. (At least I've not been able to reproduce the problem!)
As I've just launched a new course, I will keep the changes to a minimum for the next week or so. Then I'll dive in- I think the Moodle upgrade will significantly improve the user experience.
Upgrading hosting is on the agenda in the medium term.
Thx again!